Abstract

Vauchez et al. [this issue] (hereinafter refered to as VBN) interpret the petrologic, tomographic, and anisotropy data from continental rifts to support a model of continental rifting [Nicolas, 1993; Nicolas et al., 1994] in which the lithosphere splits along the rift axis and asthenosphere flows in from the sides to fill the resulting gap. We suggest here that the data can also be described by a model in which the lower lithosphere is modified or eroded by active mantle upwelling over a region of significantly greater dimensions than the rift graben and that partial melt developing in the upwelling region can account for the widespread volcanism, as well as the seismic properties. Nicolas [1993] argued that rift-aligned anisotropy could be explained by rift-parallel mantle flow. We thank VBN for bringing this relevant paper to our attention.

Volcanism about the East African Rift and the Rio Grande is not confined to the rifts but extends hundreds of kilometers from the rift axes (Mount Kilimanjaro, Mount Elgon, Mount Kenya in East Africa, The Jemez Lineament on the Rio Grande) in regions uplifted relative to their surroundings. The low-velocity tomographic anomalies also extend beneath the uplifted regions and are thought to be related to the uplift possibly supporting it by thermostatic buoyancy. The size of the P and S velocity contrasts and attenuation of high frequencies have led to the suggestion that large regions of the anomalous bodies have temperatures at or above the solidus [Achauer et al, 1994; Slack et al., 1994, 1996]. The wide extent of the anomalous regions is not explicable as resulting from an abyssal lithospheric dike beneath the rift intruded by asthenosphere. The extension of the East African, Baikal, and Rio Grande rift grabens has been estimated to be about 10 km [Baker et al., 1972; Baldridge et al., 1984; Morgan and Golombek, 1984; Logatchev and Florensov, 1978]. Passive influx of asthenosphere into a 10 km lithospheric dike is insufficient to explain the tomographic anomalies [Davis, 1991]. In addition, the amount of finite strain from lithospheric diking is insufficient to explain the anisotropy anomalies. Active replacement or modification of lower lithosphere either prior to, or contemporaneous with, rifting could generate tomographic anomalies of this magnitude.